A polypropylene material has a high mechanical strength and an excellent heat resistance, and therefore is used in a variety of fields. As a method for further improving the mechanical strength of a polypropylene material, a method for enhancing the crystallinity of a polypropylene material is known for example. In addition, as a method for further improving the heat resistance of a polypropylene material, a method for raising the melting peak temperature of a polypropylene material by making the molecular chains in a polypropylene material oriented by means of stretching the polypropylene material or the like is known for example.
For example, Patent Literature 1 proposes that a polypropylene preform having a melting peak temperature Tm (° C.) as determined by differential thermal analysis at a temperature rise rate of 20° C./min is heated to a temperature in the range from Tm−15 (° C.) to Tm (° C.) for heat treatment, thereby improving the stiffness and heat resistance of the polypropylene molded body.